write() writes up to count bytes from the buffer starting at buf to
the file referred to by the file descriptor fd.
The number of bytes written may be less than count if, for example,
there is insufficient space on the underlying physical medium, or the
RLIMIT_FSIZE resource limit is encountered (see setrlimit(2)), or the
call was interrupted by a signal handler after having written less
than count bytes. (See also pipe(7).)
For a seekable file (i.e., one to which lseek(2) may be applied, for
example, a regular file) writing takes place at the file offset, and
the file offset is incremented by the number of bytes actually
written. If the file was open(2)ed with O_APPEND, the file offset is
first set to the end of the file before writing. The adjustment of
the file offset and the write operation are performed as an atomic
step.
POSIX requires that a read(2) that can be proved to occur after a
write() has returned will return the new data. Note that not all
filesystems are POSIX conforming.
According to POSIX.1, if count is greater than SSIZE_MAX, the result
is implementation-defined; see NOTES for the upper limit on Linux.

On success, the number of bytes written is returned (zero indicates
nothing was written). It is not an error if this number is smaller
than the number of bytes requested; this may happen for example
because the disk device was filled. See also NOTES.
On error, -1 is returned, and errno is set appropriately.
If count is zero and fd refers to a regular file, then write() may
return a failure status if one of the errors below is detected. If
no errors are detected, or error detection is not performed, 0 will
be returned without causing any other effect. If count is zero and
fd refers to a file other than a regular file, the results are not
specified.

EAGAIN The file descriptor fd refers to a file other than a socket
and has been marked nonblocking (O_NONBLOCK), and the write
would block. See open(2) for further details on the
O_NONBLOCK flag.
EAGAIN or EWOULDBLOCK
The file descriptor fd refers to a socket and has been marked
nonblocking (O_NONBLOCK), and the write would block.
POSIX.1-2001 allows either error to be returned for this case,
and does not require these constants to have the same value,
so a portable application should check for both possibilities.
EBADF fd is not a valid file descriptor or is not open for writing.
EDESTADDRREQfd refers to a datagram socket for which a peer address has
not been set using connect(2).
EDQUOT The user's quota of disk blocks on the filesystem containing
the file referred to by fd has been exhausted.
EFAULT buf is outside your accessible address space.
EFBIG An attempt was made to write a file that exceeds the
implementation-defined maximum file size or the process's file
size limit, or to write at a position past the maximum allowed
offset.
EINTR The call was interrupted by a signal before any data was
written; see signal(7).
EINVAL fd is attached to an object which is unsuitable for writing;
or the file was opened with the O_DIRECT flag, and either the
address specified in buf, the value specified in count, or the
file offset is not suitably aligned.
EIO A low-level I/O error occurred while modifying the inode.
This error may relate to the write-back of data written by an
earlier write(2), which may have been issued to a different
file descriptor on the same file. Since Linux 4.13, errors
from write-back come with a promise that they may be reported
by subsequent. write(2) requests, and will be reported by a
subsequent fsync(2) (whether or not they were also reported by
write(2)). An alternate cause of EIO on networked filesystems
is when an advisory lock had been taken out on the file
descriptor and this lock has been lost. See the Lost locks
section of fcntl(2) for further details.
ENOSPC The device containing the file referred to by fd has no room
for the data.
EPERM The operation was prevented by a file seal; see fcntl(2).
EPIPE fd is connected to a pipe or socket whose reading end is
closed. When this happens the writing process will also
receive a SIGPIPE signal. (Thus, the write return value is
seen only if the program catches, blocks or ignores this
signal.)
Other errors may occur, depending on the object connected to fd.

The types size_t and ssize_t are, respectively, unsigned and signed
integer data types specified by POSIX.1.
A successful return from write() does not make any guarantee that
data has been committed to disk. On some filesystems, including NFS,
it does not even guarantee that space has successfully been reserved
for the data. In this case, some errors might be delayed until a
future write(2), fsync(2), or even close(2). The only way to be sure
is to call fsync(2) after you are done writing all your data.
If a write() is interrupted by a signal handler before any bytes are
written, then the call fails with the error EINTR; if it is
interrupted after at least one byte has been written, the call
succeeds, and returns the number of bytes written.
On Linux, write() (and similar system calls) will transfer at most
0x7ffff000 (2,147,479,552) bytes, returning the number of bytes
actually transferred. (This is true on both 32-bit and 64-bit
systems.)

According to POSIX.1-2008/SUSv4 Section XSI 2.9.7 ("Thread
Interactions with Regular File Operations"):
All of the following functions shall be atomic with respect to
each other in the effects specified in POSIX.1-2008 when they
operate on regular files or symbolic links: ...
Among the APIs subsequently listed are write() and writev(2). And
among the effects that should be atomic across threads (and
processes) are updates of the file offset. However, on Linux before
version 3.14, this was not the case: if two processes that share an
open file description (see open(2)) perform a write() (or writev(2))
at the same time, then the I/O operations were not atomic with
respect updating the file offset, with the result that the blocks of
data output by the two processes might (incorrectly) overlap. This
problem was fixed in Linux 3.14.

This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2018-02-02 WRITE(2)